A successful improved design of the basic power steering gear with the distributing valve in the piston was invented several years ago and is shown in the R. H. Sheppard U.S. Pat. No. 4,088,063, issued May 9, 1978. This improved design provides a reduced size and weight power steering gear to meet the ever increasing demands for efficiency among automotive manufacturers, and more specifically, among truck manufacturers using the Sheppard Power Steering System. The new design includes not only the space and weight-saving features but also new lubricating features providing for substantial increases in operation efficiency. In achieving the improved operation, the new gear also provided for greater manufacturing efficiency.
One feature that provided for greater operation efficiency in this earlier design was the concept of positioning the valve more in the center of the piston and providing the valve guide slot at the end of the valve adjacent the threads that engage the input shaft. Further, the slot is substantially aligned opposite the output rack so that deflection forces within the piston under heavy steering loads do not adversely affect the ability of the valve to reciprocate. This reciprocal valve movement or reversibility of the valve is one of the major parameters in gauging the efficient operation of a power steering gear. The Sheppard gear has historically excelled in this area of power steering operation. Essentially, the reposition of the valve guide slot and the better lubrication provided much better reversibility than had heretofore been attainable.
In the prior design, the guide pin included one pair of flattened, parallel guide surfaces to engage the sides of the groove. The flattened surfaces are straight with respect to the line of travel of the valve, and thus allow the valve to reciprocate but not rotate. With the proper guide pin fit, no more than approximately 0.001 inch clearance is desired.
However, even with the improvement made in the 4,088,063 patent, there was still something left to be desired, especially when the valve within the piston is hardened. It was found that in a significant number of valves after heat treating, the desired fit was not readily obtained. An inordinate amount of time was spend during the manufacturing process to handfit the guide pins to the particular valve to give best results. To compound the problem, it was found that it could not be predicted whether the slot in the valve would "grow" or "shrink" when the heat treatment was applied. Because of this, the slot could not simply be made either larger or smaller depending on the desired final size of the slot desired.
The thing that appeared to be necessary was to devise a way of adjusting the fit of the pin in the slot of the valve so that any manufactured pin could fit any valve. With such an arrangement, regardless of whether the valve slot has changed during heat treatment would not matter, since with an adjustment in the final assembly the desired tolerance could be obtained. However, insofar as I'm aware, prior to the present invention, there was not such successful adjusting means available.